Detecting a physiological state of a driver contributes to the deterrence of an accident due to the deterioration of the physiological state. Therefore, there is a detection method to detect a driver's physiological state using a state of driver's pulse or heartbeat. For example, a device that implements the detection method measures a potential difference signal between two electrodes in contact with a driver, and identifies a heart rate signal indicating driver's pulse or heartbeat from the measured potential difference signal. Then, the device detects the sleepiness or alertness, etc. as a physiological state of the driver using the identified heart rate signal.
A potential difference signal measured by the device that implements the detection method includes noise generated from other electronic devices installed in a vehicle and noise generated around the vehicle, and a heart rate signal included in the potential difference signal is weak. Therefore, the device that implements the detection method amplifies a potential difference signal by a predetermined amplification factor, and then identifies a heart rate signal.
To identify a weak heart rate signal, it is better to measure a potential difference signal using two positions resulting in a large potential difference as possible. Therefore, the device that implements the detection method measures a potential difference signal between electrodes located at two positions across the heart. The two positions across the heart are, for example, a combination of the right hand and the left hand, a combination of the right hand and the seat (the buttocks), a combination of the left hand and the seat, and the like.
For example, a plurality of electrodes to be contacted by a driver is installed in a steering part (a wheel) and a driver's seat, etc. of a vehicle. The electrode installed in the seat is contacted by driver's buttocks when a driver sits in the seat, and the electrode installed in the wheel is contacted by driver's hand when the driver holds the wheel, and then the device that implements the detection method measures a potential difference signal between the two electrodes.    Patent document 1: Japanese Laid-open Patent Publication No. 2008-237379